1. Field of the Invention
The present invention relates to an image processing apparatus capable of estimating and correcting color data which vary according to the illuminating light, and a method and a recording medium therefor.
2. Related Background Art
Utilizing the spectral reflectance R(λ) of a reflective object, the spectral distribution P(λ) of the illuminating light and the isochromatic functions: x(λ), y(λ), z(λ)  Athe colorimetric values (X, Y, Z; three stimulation values) of the reflective object under certain illuminating light can be represented by:X=k∫visR(λ)·P(λ)· x(λ)dλY=k∫visR(λ)·P(λ)· y(λ)dλZ=k∫visR(λ)·P(λ)· z(λ)dλ  Bwherein the constant k is represented by:k=100/∫visP(λ)· y(λ)dλ  Cand the integration:(∫vis)  Dis executed within the visible wavelength range.
Consequently the calorimetric values of the reflective object vary according to the variation of the illuminating light. Also according to the definitions described above, the spectral distribution P(λ) of the illuminating light and the spectral reflectance distribution R(λ) are required in order to obtain the exact calorimetric values in such state.
For this reason, the colorimetric values of the reflective object under arbitrary illuminating light have conventionally been determined from the spectral reflectance R(λ) of the reflective object and the spectral distribution P(λ) of the illuminating light.
The above-mentioned method can be easily executed and can determine the exact calorimetric values in case the reflective object consists of areas of several colors (spectral reflectance R(λ)). On the other hand, in case the reflective object is for example an image, the object in general has color information in each of finely divided many areas (pixels). Consequently there is required a large memory capacity for storing spectral reflectance R(λ) for each pixel, and the information ordinarily held for each pixel is the chromaticity values (X, Y, Z) under a specified condition (specifying illuminating light or colorimetric method) or equivalent RGB chromaticity values. In order to determine the calorimetric values of the reflective object for the arbitrary illuminating light in the above-described method, there is required the spectral reflectance R(λ) for each pixel, so that the spectral reflectance R(λ) is determined again from the information corresponding to the aforementioned colorimetric values (X, Y, Z) for each pixel or the spectral reflectance R(λ) is measured again for each pixel.
In case the information obtained for each pixel is the values corresponding to the colorimetric values (such as the aforementioned XYZ values of RGB values) under a specified condition, the colorimetric values of the reflective object under arbitrary illuminating light can be determined, in addition to the above-described method according to the foregoing definitions based on the spectral reflectance R(λ) of the object and the spectral distribution P(λ) of the illuminating light, by a method of directly converting the information for each pixel, corresponding to the colorimetric values under the specified condition, into the colorimetric values under the arbitrary illuminating light utilizing a matrix, a three-dimensional look-up table or a neural network. The conversion function (above-mentioned matrix, three-dimensional look-up table or neural network) is determined for each of the required plural illuminating lights.
As explained in the foregoing, the calorimetric values of the reflective object vary according the change in the illuminating light. In an image reproducing process or the like, there are often required the calorimetric values of the reflective object under arbitrary illuminating light.
In case the information obtained for each pixel is the values corresponding to the colorimetric values (such as the aforementioned XYZ values of RGB values) under a specified condition, the calorimetric values of the reflective object under arbitrary illuminating light can be determined by a method of directly converting the information for each pixel, corresponding to the calorimetric values under the specified condition, into the calorimetric values under the arbitrary illuminating light utilizing a matrix, a three-dimensional look-up table or a neural network. The conversion function (above-mentioned matrix, three-dimensional look-up table or neural network) is determined for each of the required plural illuminating lights.
The above-described method is acceptable in case the number of the required illuminating lights is limited. However, for example in the ordinary office environment, the condition of lighting changes in various manner according to the kind of the illuminating light source, the time-dependent change thereof, and the change in the state of the incoming external light such as the solar light, and it is difficult to prepare or store in advance the conversion functions required corresponding to such changes.
As explained in the foregoing, in order to determine the calorimetric values under arbitrary illuminating light for an image or the like that requires information for each of a large number of pixels, the conventional methods have been associated with drawbacks of requiring a large amount of information such as preparing the spectral reflectance for each pixel or preparing a large number of conversion functions corresponding to various illuminating light conditions.
On the other hand, owing to the recent commercialization of various color image processing equipment, the color images can be handily processed not only in the special fields such as designing based on the computer graphics but also in the ordinary offices. It has however been difficult to consider the color of the printout on the monitor, because the color of the image prepared on the monitor does not in general match that of the printout obtained from the printer. In order to solve such drawback, there has been considered and proposed the color management system.
The color management system is to cancel the difference in color between the different devices by employing a common color space. This system is based on a principle that a color described by same coordinate values in a same color space should always look same and is to match the apparent color by representing all the colors in a common color space and matching the coordinate values. One of the currently employed methods for canceling the difference between the devices employs the CIE-XYZ color space and utilizes the XYZ stimulation values which are the internal description coordinates therein.
However such method may still be insufficient in case the media used for reproduction are different, for example the image on a monitor and the printed image. FIG. 27 shows the environment for observing an image on a monitor and a printout. In the following it is assumed that an image 1202 same as that on a print 1201 is displayed on a monitor 1203.
The printed image or the image displayed on the monitor is not observed under constant ambient light, but the ambient light 1204 shown in FIG. 27 changes by the opening or closing of a window or the replacement of the illuminating light source, and the image appears differently by such change. Consequently, even when isochromaticity can be obtained under certain ambient light, it cannot be preserved even under the same conditions in case the ambient lighting condition is altered.
Though the foregoing consideration has been explained in the comparison of the printed image and the image displayed on the monitor, it is generally applicable to the comparison of a color presentation by reflecting illuminating light and a light-emitting color presentation. More specifically, such phenomenon occurs also in case of taking an object such as a person or a sculpture and displaying it on a monitor or presenting as a transmissive display.
As explained in the foregoing, the appearance of the image varies according the change in the image observing environment. In the conventional art employing different media, the images providing isochromaticity under a certain situation no longer provide isochromatic feeling by the change in the image observing environment.